If you don’t have lsof already you can download and install it by becoming root and running:

root@mystery:~# apt-get install lsof
This will download and install the package for you, along with any dependencies which might be required:

If you have the Apache webserver running on port 80 that will provide a suitable test candidate. If not you can choose another port you know is in use.

To discover the process name, ID (pid), and other details you need to run:

lsof -i :port

E.g. to see which process is listening upon port 80 we can run:

root@mystery:~# lsof -i :80
This gives us the following output:

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
apache2 10437 root 3u IPv6 22890556 TCP *:www (LISTEN)
apache2 10438 www-data 3u IPv6 22890556 TCP *:www (LISTEN)
apache2 10439 www-data 3u IPv6 22890556 TCP *:www (LISTEN)
apache2 10440 www-data 3u IPv6 22890556 TCP *:www (LISTEN)
apache2 10441 www-data 3u IPv6 22890556 TCP *:www (LISTEN)
apache2 10442 www-data 3u IPv6 22890556 TCP *:www (LISTEN)
apache2 25966 www-data 3u IPv6 22890556 TCP *:www (LISTEN)
apache2 25968 www-data 3u IPv6 22890556 TCP *:www (LISTEN)

How to find out which process is listening upon a port.

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